1. Field of the Invention
The invention relates to a method for the preparation of solid carrier particles for a polymerization catalyst, in which there is provided a melt of a complex composition having the formula (I) EQU MgCl.sub.2 nROH mED (I)
in which ROH depicts an aliphatic alcohol, R is a C.sub.1 -C.sub.6 alkyl, ED depicts an electron donor, n is 1 to 6 and m is 0 to 1; the melt provided is fed to a nozzle; the melt is sprayed from the nozzle to a spraying area in which it is divided into fine melt droplets and solidifies to solid carrier particles; and the solid carrier particles are recovered. The invention also relates to a method in which the olefin polymerization catalyst is provided by bringing the recovered solid carrier particles together with a transition metal compound being catalytically capable of activation, such as titanium tetrachloride, and the use of the olefin polymerization catalyst thus prepared together with a cocatalyst and alternately an external electron donor for the polymerization of olefins.
2. Description of the Related Art
Polymerization catalysts and particularly catalysts of Ziegler-Natta type nowadays typically comprise an inert solid carrier, on which the actual active catalyst component or the mixture or complex formed by the catalytical compounds is layered. The chemical composition of the particles, the structure of the surface, the morphology, the particle size and the particle size distribution of such a carrier are of major significance for the activity of the catalyst and the properties of the polymer to be obtained. With a very active catalyst, namely, polymer can be produced from which thanks to its purity no catalyst residues need to be removed. The surface structure and the morphology of the carrier, on the other hand, affect on the morphology of the polymerization product itself, for it has been noticed that the morphology of the catalyst is repeated in the structure of the polymer (the so-called replica phenomenon). When the aim is a flowing product polymer having the desired morphology and a narrow particle size distribution, which is desirable in view of the objects of use of many of the processing processes, the properties of the carrier should, because of the replica phenomenon, correspond to those of the desired polymer particle.
The catalysts of the above-mentioned type are nowadays often formed of magnesium based carrier substance, which has been treated with a transition metal compound like titanium halide and often also with an electron donor compound. It is also known that a carrier can be brought into a preferred chemical composition having a certain surface structure, a certain morphology, a certain particle size and a certain particle size distribution by letting it crystallize as a complex of one of its crystal solvents.
In the method according to EP publication 65,700 and U.S. Pat. No. 4,421,674 the titanium halide is brought to react with a magnesium chloride catalyst carrier being in the form of micro balls, after which the reaction product particles are recovered by physical means and are mixed together with an organometallic compound. In this method the carrier is prepared by providing a solution, which essentially contains magnesium dichloride dissolved in ethanol and a spray-drying of the solution is carried out by spraying it into a flow of nitrogen gas, the inlet and outlet temperatures of which are high. As a result magnesium dichloride particles of very even size having the form of a ball are obtained. In this method the high temperature evaporates, however, a great deal of the crystallization solvent, whereby porosity is created on the surface of the carrier and its activation capability decreases. This leads to a solid catalyst having a satisfactory particle size distribution, but the activity and mechanical strength of the catalyst is poor due to the porosity.
In the FI-patent application 862459 (Neste Oy) there is disclosed a method for the preparation of a carrier, in which there does not appear any porosity decreasing the activity and the mechanical strength.
In the method the carrier complex formed by the carrier and the crystal solvent is melted to a clear liquid. When the liquid is conducted through the nozzle and the spraying space into the crystallizing space cooled with cold nitrogen gas the carrier complex crystallizes to small particles having a spherical form which are very flowing and loose. Furthermore, the carrier complex crystallizes without considerable evaporation of the crystal solvent. Hereby a nacreous non-porous surface is obtained, which is particularly well suitable for the preparation of an active catalyst. When such a preactivated carrier is brought into contact with a titanium compound, abundantly of catalytically active complexes between the MgCl.sub.2 and the titanium compound, are formed onto the surface of the carrier, when the crystal solvent leaves.
The above-mentioned Finnish method has in result easily activating carrier compounds and complexes. A drawback of this so-called spray-crystallizing method is, however, that the droplets formed are not of fully equal size and that they are partly agglomerated. Although, as to their surface structure, more useful carrier particles are obtained by this method than e.g. by spray-drying, the problem thus is that an unsatisfactory particle size distribution is formed and that the particles are partly agglomerated.